[cs_themes] recover from corrupted icon cache

[sean0407]

Summary

- Parse every cache line as exactly one theme:path pair.
+ Skip damaged entries, preserve colons in paths, and rebuild a damaged cache.

Evidence

• Before: Invalid UTF-8 raises UnicodeDecodeError; a line without : raises ValueError while opening the Themes panel (cs_themes.py crashes on corrupted theme-cache file (UnicodeDecodeError / ValueError / UnboundLocalError) #14035).
• After: The cache reader skips malformed entries and flags the cache for rewriting. The focused tests cover invalid UTF-8, malformed and empty entries, embedded control bytes, paths containing colons, and valid caches. Both tests pass.
• py_compile and git diff --check pass.

Merge Danger

Door: Two-way

Blast Radius: Low

Only the Cinnamon Settings icon-theme cache reader and its focused tests change.